Planetary Atmospheres

Planetary Atmospheres

H Chandra Physical Research Laboratory Ahmedabad 380 009, India

1st Asia Pacific School on International Heliophysical Year (10-22 December 2007) Kodaikanal Observatory Indian Institute of Astrophysics, Banglore, India

Planetary Atmospheres 1. The Planets There are eight planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune (mercury being the nearest and Pluto farthest from the Sun) that revolve around Sun in their specific orbits, which lie more or less in the Sun’s equatorial plane. Then there are moons or natural satellites, which revolve the central bodies, i.e. planets. It is natural to think that planetary bodies have evolved from the Sun and the moons from their central bodies. However earth’s moon has been found to be older than earth and has its own history of evolution. The biggest planet Jupiter is more akin to Sun than to other planets (more like a star). In fact Mercury, Venus and Mars show surface features similar to our moon. The planets can be divided into two categories. 1. The inner planets comprising of Mercury, Venus, Earth and Mars which have densities of the order of 5 or more and sizes comparable to that of earth. 2. The outer planets (Jupiter, Saturn, Uranus and Neptune), which are quite large in size and have low densities ≈ 1.5 (Jupiter like hence called Jovian planets).

Table 1: Planetary Data Planet

Mean radius km

Mean density gmcm3

Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune

2439 6050 6371 3390 69500 58100 24500 24600

5.42 5.25 5.51 3.96 1.35 0.69 1.44 1.65

Average distance from Sun AU 0.39 0.72 1.00 1.52 5.2 9.5 20 30

Length of Rotation year- days perioddays

Inclination degree

88 225 365 687 4330 10800 30700 60200

ve.

7.Diffusion Effect of turbulent mixing or molecular diffusion (due to the gradients in relative concentration as a result of slight deviations from Maxwellian distribution) is to transport them with a vertical flux. Diffusion becomes more important at higher altitudes as collisions decrease. Vertical drift velocity due to pressure gradient is given by w = -(D/n). ∂n/∂z

[D is the diffusion coefficient]

(7.1)

Also ∂p/∂z = kT ∂n/∂z Equating this to the drag force due to the collisions nmνw kT ∂n/∂z = - nmνw

(7.2)

Combining the two equations we get the diffusion coefficient given by D = kT/mν

(7.3)

Collision frequency decreases with height so the diffusion coefficient increases. As ν is proportional to nT1/2 D ∝ T1/2 n-1 For vertical motion gravitational force must be included. Therefore nmνw = -dp/dz – nmg

(7.4)

The values of p, D and scale height of neutral atmosphere Hn are p = nkT, D = kT/mν and Hn = kT/mg Hence nw = -D [(dn/dz) + (n/Hn)]

(7.5)

Therefore w = -(D/n) [(dn/dz) + (n/Hn)] = -(D/n).(dn/dz) – D/Hn

(7.6)

Including the effect of turbulent mixing the vertical flux can be described by φi = -(Di + K) [ dni/dz) + (ni/T) (dT/dz)] – ni [Di/Hi + (K/H)]

(7.7)

Here Di is the diffusion coefficient for the ith species and K is the eddy diffusion coefficient. Further φi = 0 under equilbrium condition. When Di